occasional sewer gas smell in laundry room

First off, thanks for all the help so far on this site- it has proven most valuable to us!

Our house is a basic square proportioned cottage with a 7' basement ceiling and concrete slab floor. Our laundry room was added on by a previous owner- the floor of the laundry room might be an inch higher than the rest of the basement, and it has a doorway through the old concrete foundation.

All drain pipes mentioned are 2" black PVC.

The image shown is roughly how the drain pipe looks from the exposed concrete wall in laundry room. The pipe extends to somewhere on the "inside" of the house, and is routed through the concrete foundation about 12" from the floor. There is one branch with a trap that has perhaps 1'-2' of vertical that connects to the washer drain tube. The drain pipe extends into the ceiling (we have removed sheetrock around it so we can see what is there) and has what looks like a vent of some sort on it. The piece is plugged into the pipe's end, and is beige in color. It doesn't stink in the laundry room often, just occasionally.

Does this seem like some improperly vented drain pipe? Without knowing what else is down the line from the drain, would it be advisable to cap the pipe in the ceiling?

-ben

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occasional sewer gas smell in laundry room

It sounds like you may have an air admittance valve in lieu of an actual open vent pipe at the top. However, that shouldn't be causing sewer gas to leak back.
Do you know if there is a floor drain? Sometimes the standing water in a floor drain trap will evaporate, allowing sewer gas back in. The simple solution is to pour water down the floor drain on a regular basis.
Do NOT cap the pipe in the ceiling.
Mike

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occasional sewer gas smell in laundry room

We have no floor drains in the basement whatsoever.

Is it possible that the higher pressure of the washing machine outlet might push water out of the trap, then when a nearby toilet is flushed it causes just enough pressure difference in the pipes that some gas escapes? Or might the valve be defective?

occasional sewer gas smell in laundry room

No matter what the pressure of the washer discharge, it should leave the trap filled to prevent sewer gas after it shuts off.
If there is some partial blockage of a vent pipe through a roof (leaves, debris, birdnest, etc.) it can cause a nearby trap to siphon out allowing sewer gas in after water is run through the drain pipes.
Check the vent pipe(s) on the roof. Clear them of any visible debris by hand and flush them down with a garden hose sprayer to clear them. Sometimes, that's all that's needed.
However, you apparently have an air admittance valve at the top of that pipe in lieu of a regular vent pipe connection, so you may need to cut the old one off and replace that.